I've come a across a document by Linear Technologies that suggests several circuit schematics to interface with an Avalanche Photodiode (APD). They are referred to as current monitors and placed before the APD (in the schematic, in practical terms it is irrelevant). Aren't these the same as transimpadance amplifiers? If not, what exactly are the differences/advantages/disadvantages, specially compared to the approaches suggested in this answer ?

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1 Answer 1


Aren't these the same as transimpadance amplifiers?

Well, yes and no. Technically, any current to voltage converter can be called a transimpedance amplifier. However, when dealing with opamps this generally refers to a configuration where the current is injected directly into one of the opamp inputs and the feedback resistor does the voltage conversion. The answer you link to is a good example. The big problem with this approach is that the conditioning electronics is tightly constrained by the need to return all the current through the load into the amplifier. For multiple sensors this can be difficult.

The current monitor, as shown above, isolates the sense element from the amplifier. This has good and bad consequences. On the plus side, it's easier to measure multiple sensors, and good frequency response is easier to guarantee. On the minus side, you have to insert a separate monitor element into the current path, and the usable gain is typically a good deal less. Also, the transimpedance configuration potentioally injects noise directly into the opamp, which can cause unexpected and intermittent problems.

Which approach one chooses depends on which considerations are more important.

  • \$\begingroup\$ So for a single sensor and low-noise measurement, a conventional transimpedance amplifier is recommended. \$\endgroup\$
    – joaocandre
    May 15, 2014 at 9:32
  • \$\begingroup\$ Only as long as you need a lot of gain and the speed requirements are not too great. For instance, lab photodiodes used for monitoring high speed pulses commonly use a battery-biased photodiode which feeds a coax cable, and the current detection is performed at the other end of the coax by a 50 ohm resistor. See, for instance, Thorlabs. \$\endgroup\$ May 15, 2014 at 13:08

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